Field of the Invention and Related Art Statement
This invention relates to an imaging system for providing an amplified electrical image signal for input to a visual display, wherein heat buildup in the amplification of the electrical image signal is inhibited by a reduction in the power consumption of an imaging means during periods when no readout of the electrical image signal occurs.
Recently there are suggested various imaging apparatuses each using a solid state imaging device such as a CCD (charge coupled device). There are also suggested various endoscopes each using said solid state imaging device as a special imaging apparatus. In such endoscopes, there is known a method in which a color filter is provided integrally with or separately from the CCD and a method in which three primary color lights are radiated as sequentially switched to the CCD. Their detailed formations are mentioned, for example, in the publications of Japanese Patent Application Laid Open Nos. 65962/1976 and 54933/1980.
Also, the actual fitting around the solid state imaging device of such apparatus and the transmission of an output signal therein are mentioned in detail, for example, in the publications of Japanese Utility Model Application Publication No. 19122/1977 and Japanese Patent Application Laid Open No. 61588/1986.
FIG. 1 shows an electronic endoscope apparatus 1 of a prior art example similar to the one disclosed, for example, in Japanese Patent Application Laid Open No. 65962/1976.
This electronic endoscope apparatus 1 comprises an electronic scope 2, a video processor (called also an apparatus body) 5 having a light source section 3 feeding an illuminating light to this electronic scope 2 and a signal processing section 4 processing the signal for the imaging means of the electronic scope 2 built-in and a color monitor 6 displaying a standard video signal produced by processing the signal in the signal processing section.
Said electronic scope 2 has an elongate insertable section 7 and a thick operating section formed at the rear end of this insertable part 7. A light guide 9 transmitting the illuminating light is inserted through the insertable section 7 so that, when the light guide 9 extended out of the operating section 8 is fitted at the end to the light source section 3, the illuminating light may be fed from the light source section 3.
That is to say, when a white light emitted from a lamp 11 is condensed by a lens 12 and is passed through a rotary filter 14 rotated and driven by a motor 13, color transmitting filters 15R, 15G and 15B transmitting the lights of respective wavelength regions of red (R), green (G) and blue (B) and fitted in the peripheral direction of this rotary filter 14 will be sequentially interposed into the light path and the light will be thereby converted to RGB sequential lights. These RGB (sequential) lights are condensed by a lens 16 and are radiated to the light guide 9 on one end surface. The RGB lights are transmitted by this light guide 9 and are emitted to the object side through an illuminating lens 18 from the end surface on the scope tip section 17 side.
The reflected light from the object is made by an objective lens 19 fitted to the scope tip section 17 to form an object image on a CCD 21 as a solid state imaging device arranged in its focal plane. The object image is photoelectrically converted by the CCD 21 and is stored as an electric charge corresponding to the object image.
In said CCD 21, when a CCD driving signal from a CCD driver 22 within the video processor 5 is applied, an electric charge will be read out. This read-out signal is amplified in the current by a buffer amplifier 23 and is transmitted to a pre-processing circuit 25 within the video processor 5 through a transmitting cable 24 inserted through the scope, that is, the insertable section 7 and the cord extended out of the operating section 8.
In this pre-processing circuit 25, a base band signal is extracted from a signal output from the CCD 21 as synchronized with a horizontal transfer clock of a CCD driving signal and gamma not illustrated within the pre-processing circuit 25 is corrected. This signal having had the gamma corrected is converted to a digital signal by an A/D converter 26 and is sequentially written by a control signal of a controlling section 28 into R, G and B memories 27R, 27G and 27B for synchronization.
For example, the signal photoelectrically converted by the CCD 21 under the illuminating light of red having passed through the red color transmitting filter 15 is written into the R memory 27R.
The respective signals temporarily written into these memories 27R, 27G and 27B are simultaneously read out and are converted to synchronized digital RGB signals which are converted to standard analogue RGB signals by D/A converters 29R, 29G and 29B. These RGB signals are input into enhancing circuits 31R, 31G and 31B for visibly improving the sharpness, are enhanced in the outlines and are then output to a color monitor 6 through buffer amplifiers 32R, 32G and 32B to color-display the object image.
By the way, a synchronizing signal is input into the controlling section 28 from a synchronizing signal generator 33 and the A/D conversion of the A/D converter 26, the reading/writing of the memories 27R, 27G and 27B, the D/A conversion of the D/A converters 29R, 29G and 29B, the rotating speed of the motor 13 and the timing of the driving signal of the CCD driver 22 are controlled as synchronized with this synchronizing signal.
Generally, in the electronic endoscope apparatus including the prior art example shown in this drawing, various parts are diagnosed by inserting the electronic endoscope 2 into a body cavity or are treated by passing a treating instrument or the like through a channel not illustrated provided within the insertable section 7.
So that these functions may be well developed, the outside diameter and rigid part length of the tip section 17 are required to be small and short and therefore the size of the solid state imaging device which can be used is restricted. From this restriction, the number of pixels of the solid state imaging device is also restricted. Also, in order to visibly display such required information as patient data, an object image that is imaged with the solid state imaging device is not displayed on the whole screen at observation monitoring, but the same as in the usual case of this prior art example, a display size of about 1/2 is used as shown in FIG. 2. By the way, said patient data are simultaneously displayed on the left side in the picture in FIG. 2.
Also, in this prior art example, a so-called frame sequential system wherein three R, G and B primary color lights are sequentially radiated by using a black and white (monochromatic) solid state imaging device (specifically the CCD 21) is adopted. In order to provide a distance range in which the observation can be made with a proper brightness, it is desirable to enlarge the illuminating light amount per frame/field by lengthening the illuminating period. Signals are read at a high speed out of the CCD 21 and the signals read out by shortening the reading period are temporarily written into the R, G and B memories 27R, 27G and 27B for synchronizing the signals read out and are read at a standard video signal speed out of said R, G and B memories 27R, 27G and 27B.
Now, the transmission system of the output signal of the CCD 21 within the scope 2 in FIG. 1 is as in FIG. 3.
The length of the transmitting cable 24 from the scope tip section 17 to the video processor (body apparatus) 5 is usually about 2 to 3 meters. A coaxial cable 24 is used for the transmission for this length with as little deterioration of the characterisitcs as possible. A buffer amplifier 23 comprising an emitterfollower-connected transistor 34 arranged just adjacently to the CCD 21 and an emitter resistance R1 is necessary to drive the readout signal from the CCD 21 through this coaxial cable 24.
The signal output end of the CCD 21 is connected to the base of the transistor 34. The power source end of the CCD 21 and the collector of the transistor 34 are connected to a power source end +Vcc through a power source cable 35.
The characteristic impedance of said coaxial cable 24 is usually about 50 to 75.OMEGA.. The resistance R1 interposed between the emitter and one end of the coaxial cable 24 and the resistance R connected between the end part of the coaxial cable 24 on the apparatus body 5 side and ground are matching resistances.
Now, in order to transmit the output signal of the CCD 21 having a band characteristically of several MHz through the cable of 50 to 75.OMEGA., it is necessary to flow an electric current of several mA to several ten mA through the transistor 34 forming the buffer amplifier 23. The power consumption by this current becomes the sum of the consumptions by the transistor 34 and emitter resistance R1, that is, P=Ic.times.Vce+Ie.times.R1 and the generated heat amount brings about a temperature rise of the just adjacently arranged CCD 21. Here, Ic represents a collector current of the transistor 34, Vce represents a voltage between the collector and emitter and Ie represents an emitter current.
Generally, with the temperature rise, a solid state imaging such as the CCD 21 increases the dark current and deteriorates the picture quality. Usually, the operation guaranteeing temperature of this solid state imaging device is an absolute maximum rating of about 55.degree. C. Particularly, in the case of a plurality of horizontal transferring system solid state imaging devices of a high number of pixels, said buffer amplifier 23 will require a plurality of circuits and the temperature range of this maximum rating will be exceeded.
As described above, in the prior art example, there have been problems in that the heat generation by the driving buffer amplifier 23 of the transferring cable 24 will be large, the picture quality will be deteriorated by the increase of the dark current, the temperature range of the absolute maximum rating of the solid state imaging device will be exceeded, the life will be shortened and permanent damage will occur.